Vehicle fuel systems may include a carbon canister to adsorb fuel vapors resulting from refueling, diurnal temperature swings, heat rejection following engine shutdown, and running loss. Once the canister is loaded with vapors, engine running manifold vacuum is used to clean out the canister in a process known as purging. When a vehicle is driven on a hot day, even after the engine is shutdown, heat from the hot engine persists or even increases and can transfer to the fuel tank and cause fuel vapor generation, a process referred to as hot soak. Vapor generation from an engine hot soak is undesirable since it loads the canister. Too much loading of the carbon canister can result in breakthrough of hydrocarbons (HC) to the atmosphere.
Other attempts to manage engine heat rejection in a vehicle include operating an engine cooling fan to cool the engine when the vehicle is stopped. One example approach is shown by MacKelvie in U.S. Pat. No. 7,121,368. Therein, an engine cooling fan associated with a radiator is operated in a reverse direction when the vehicle is moving slowly or is stopped to draw outside air from beneath the vehicle and through the engine bay thereby cooling the engine surface, components in the engine bay, and the firewall of the vehicle's interior. However, the inventors herein have recognized potential issues with such systems. As one example, engine heat rejection may only lead to fuel vapor generation when the engine heat actually travels to the fuel tank, and operation of the engine cooling fan when engine cooling is not warranted wastes energy. For example, when a vehicle is parked on a grade, the engine may be positioned vertically above the fuel tank, and hence heat from the engine naturally rises away from the fuel tank. Operation of the engine cooling fan during these conditions may not reduce fuel vapor generation and thus may waste energy.
In one example, the issues described above may be addressed by a method for an engine, including, after an engine shutdown request is received, adjusting an engine cooling fan based on an engine temperature and an elevation of a fuel tank relative to the engine. In this way, by taking into account the elevation of the fuel tank relative to the engine, the engine cooling fan may be operated in reverse with the engine at rest only when the fuel tank is vertically above the engine and stopped, and hence is positioned to receive heat rising above the engine.
As one example, the cooling fan may be adjusted to operate in a reverse direction such that air surrounding the engine is forced outside of the vehicle and away from the fuel tank. Then, during normal engine operation when engine cooling is indicated, the engine cooling fan may be operated in a forward direction to direct ambient air over the engine and fuel tank. By doing so, the engine cooling fan may be operated in a direction that provides optimal air movement away from the engine or fuel tank while taking into account the vehicle grade and hence the amount of heat actually transferred to the fuel tank to the engine, thus avoiding unnecessary operation of the engine cooling fan.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.